<div class="box">
   Lorem Ipsum
   <div class="post">Lorem Ipsum</div>
</div>
<div class="box-rem">
   Lorem Ipsum
   <div class="post">Lorem Ipsum</div>
</div>
:root {
   font-size: 50px;
}
body {
   font-size: 30px;
}
.post {
   font-size: 1.2em;
}
.box-rem .post {
   font-size: 1.2rem;
}
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